The high pressure forms of boron nitride, known as cubic boron nitride ("CBN") and wurzitic boron nitride, are surpassed only by diamond in hardness and have a wide variety of uses as machining tools, abrasives, wire dies, wear parts, heat sinks, and the like.
Wurzite boron nitride, typically formed by shock or explosive techniques, has a hardness equal to CBN and can be substituted or mixed with CBN in most applications. It is thermodynamically unstable relative to CBN under conditions favorable to sintering and will revert to CBN in the presence of catalyst-solvents.
CBN, in particular, is preferred to diamond in working with ferrous metals because it is chemically more stable than diamond, has a higher temperature threshold for conversion to its hexagonal or graphitic form, and is not catalytically degraded by hot ferrous metals, as is diamond. In the applications mentioned above, the primary qualities desired for a polycrystalline compact tool are wear resistance (to increase tool life), thermal stability (to prevent failure of the tool at high temperatures), thermal conductivity (to rapidly remove excess heat from the tool and working piece), impact resistant (to allow deep cuts in a work piece and interrupted cuts), and a low coefficient of friction in contact with the workpiece. While CBN itself possesses each of these qualities to a significant degree, whether a polycrystalline compact of CBN as a whole possesses them will depend largely on the characteristics of the other materials that make up the compact--i.e., binder materials, substrates, and the like.
One particularly severe application for CBN compacts is the high-speed machining of hardened tool steels or nickel-based super-alloys. If the tool remains sufficiently sharp and maintains a low coefficient of friction in contact with the workpiece, most of the cutting energy is transferred to the immediate zone destined for removal, rather than to the tool or workpiece. As a result, only the immediate zone destined for removal is softened, and most of the heat is carried off in the extremely hot, glowing chip. In general, the highest possible volume concentration of CBN has been favored for maintaining a sharp cutting edge under these conditions.
An application presenting contrasting requirements is the rough machining of cast iron parts or for parts containing holes, slots, etc., where the interrupted cut generates high shock forces. In the prior art, compacts having a lower concentration of CBN and a higher concentration of a non-metallic binder have been favored for such applications. At present no known compact of the prior art is capable of performing well under both sets of conditions.
Various attempts have been made to produce from the high pressure forms of boron nitride polycrystalline CBN compacts which meet both of the above sets of requirements. Although it is possible to form a sintered compact of CBN with no binder material under conditions of high pressure and temperature, strongly adherent surface oxides of boron inhibit intergranular bonding and make it difficult, if not impossible, to obtain adequate compact strength. Various binder materials are thus incorporated, either to enhance intergranular bonding or to surround the grains with a continuous matrix.
The binder material should possess two general sets of qualities: (1) mechanical and chemical properties as close to those of CBN as possible, so as not to deteriorate tool performance, and (2) characteristics enabling manufacture of the compact, such as a melting point at readily obtainable temperatures or good plasticity at such temperatures, limited but not excessive chemical reactivity toward CBN, and most preferably, catalytic/solvent activity for conversion of hexagonal boron nitride (the graphitic low pressure form of boron nitride, referred to as HBN) to CBN. This latter characteristic will enable grain regrowth and intergranular bonding under conditions of pressure and temperature for which CBN is thermodynamically stable. Prior art binder compositions have, in the main, possessed significantly more attributes from one of these sets of properties than from the other. As a result, these compacts tend to exhibit characteristics more suitable either for abrasion resistance (high CBN concentrations with catalytic binders) or impact resistance (lower CBN concentration with ceramic, non-catalytic binders), but not both.
Most known catalyst-solvents for conversion of HBN to CBN are totally unsuited for use as binders because of poor mechanical properties or poor chemical stability in the presence of air, water, cooling fluids, brazing fluxes, etc. Such catalyst-solvents are taught by U.S. Pat. Nos. 2,947,617 to Wentorf, and 3,701,826 to DeVries et al., which specify alkaline, alkaline earth, tin, lead, and antimony metals and their compounds, especially nitrides and boron nitrides, and U.S. Pat. No. 3,150,929 to Wentorf, which specifies actinide or lanthanide metals.
Silicon and aluminum-containing materials, the powders and alloys of which constitute the binder of the present invention, have certain desirable properties which have led to their use, separately, in prior art binder compositions. We consider first the use of aluminum in the prior art.
The use of aluminum-ferrous metal alloys as catalyst-solvents for conversion of HBN to CBN is taught, without reference to binder applications, by H. Saito, Journal of the Ceramic Society of Japan, "Yogyo Kyokai Shi", Vol. 78, pp. 1-8, 1970 (with iron), abstracted in Chemical Abstracts, Vol. 72:139166p, and by Taylor et al., U.S. Pat. No. 3,768,972 (with iron, cobalt, or nickel). These catalyst-solvent alloys have improved hardness and strength and higher melting points than those catalyst-solvents mentioned above and are less reactive toward air. U.S. Pat. No. 3,918,219 to Wentorf et al., also discloses the use of aluminum alloys with cobalt, nickel, or manganese for making a CBN compact by means of conversion of HBN to CBN. The well-known function of aluminum in removal of surface boron oxides is taught in these specifications. This compact has poor wear resistance because the synthesis process never goes to completion, and because the shrinkage associated with conversion of HBN to CBN leaves the CBN grains completely surrounded by catalyst, preventing inter-granular bonding. Another process for making a CBN compact by synthesis (i.e., from HBN as a starting material) is U.S. Pat. No. 4,361,543 to Zhdanovich et al., which allows a wide variety of catalysts, including aluminum, silicon, their mixtures and alloys, and aluminum-cobalt with hafnium diboride additive, and specifies that pressures of 80-120 kbar and temperatures of 1800.degree.-3000.degree. C. are required for complete conversion. Such high pressures are unattainable in commercial practice since routine operation of the well-known belt or multi-anvil types of commercial high pressure equipment is limited to approximately 65 kbar. In general, compacts utilizing HBN as a starting material are inferior to those utilizing CBN crystals, and must be made at commercially impracticable conditions.
Use of transition metal-aluminum alloy catalysts for sintering of pre-existing CBN grains is taught by U.S. Pat. No. 3,743,489 to Wentorf et al. The compact of this invention, believed to be marketed under the trade name "BZN", has excellent wear resistance in machining of extremely abrasive materials as a result of high concentrations of CBN and true inter-granular bonding, but it suffers from low impact resistance, resulting in brittleness and chipping in interrupted cut applications. It additionally suffers from temperature instability, which is harmful to its performance at the highest speeds. The large difference in coefficient of thermal expansion between the cobalt-aluminum binder and the CBN phase apparently causes cracking at high temperatures as the network of metal expands against the network of CBN grains. The metal phase also has a tendency to stick to the workpiece, especially at the higher temperatures desired for machining hard alloys, thereby increasing the heating of tool or workpiece to the point of damage. Additionally, certain toolmaking processes, such as brazing with stronger, higher temperature alloys or mounting of compacts with powder metal sintering steps, must be avoided.
An example of a low pressure compact utilizing aluminum alloys with high aluminum concentration is taught by U.S. Pat. No. 4,110,084 to Lee et al., and the reissue thereof, U.S. Pat. No. Re.30,503, wherein aluminum or an alloy of aluminum with certain transition metals infiltrates the CBN mass. The binder is present in high volume concentration, and it has a low melting point, high coefficient of sticking or friction versus metallic workpieces and high thermal expansivity, thereby severely limiting tool applications.
Use of aluminum as an aid in bonding CBN to a second refractory phase under high pressure-temperature conditions is taught by U.S. Pat. No. 3,944,398 to Bell. Bell teaches the use of a binder material consisting of a boride, nitride, or silicide refractory substance and a solvent substance of aluminum, lead, tin, magnesium, lithium, or alloys thereof. Bell also teaches that the refractory substance may be a silicon nitride powder mixed with silicon, boron, and boron carbide, all in powdered form or various mixtures or reaction products of silicon nitride, alumina and/or magnesium oxide. See from Col. 1, 1. 53 to col. 2, 1. 5. The preferred embodiment of Bell, believed to be marketed under the trade name "Amborite," employs silicon nitride as the second refractory substance and aluminum as the "solvent". Bell teaches that substantially all of the aluminum reacts with the silicon nitride to form aluminum nitride and unidentified silicon compounds. This marketed compact does have good thermal stability. It performs well in aggressive cutting operations of hard ferrous alloys wherein the small portion of the workpiece which encounters the tool is heated to softening temperatures. However, the relatively high content of the binder compounds, which are considerably softer than CBN thereby imparting the desirable impact-resistance, interferes with true intergranular bonding and makes this compact less abrasion resistant than the marketed "BZN" compact, because the individual CBN grains are most subject to pull-out by the abrasive particles of the workpiece.
Use of pure aluminum as a binder for CBN is taught by UK patent application GB No. 2,048,927A, Mar. 18, 1980, to Wilson. Here also substantially all of the aluminum is taught to be converted to aluminum nitride and/or aluminum diboride by reaction with CBN. Although the compact of this UK application exhibits increased brittleness and therefore less impact resistance than the silicon nitride-aluminum nitride bonded compact of Bell, its greater CBN concentration and resulting improved thermal conductivity and thermal resistance is evidenced by the turning test of example 7 of this UK application, in which the product of that invention performed better in aggressive turning of hardened tool steel than compacts produced according to U.S. Pat. No. 3,944,398 to Bell and U.S. Pat. No. 3,743,489 to Wentorf et al.
U.S. Pat. No. 4,343,651 to Yazu et al., teaches that the CBN content may be increased to more than 80 volume percent by adding aluminum compounds to the binder material which is a carbide, a nitride, or a carbonitride of a group IVb and Va transition metal. This provides a compact more suited for abrasive wear applications; cutting of sintered WC/Cobalt of 15 wt % cobalt is given as one example. A substantial quantity of aluminum (up to 20 weight percent of the binder) is added to facilitate sintering, as plastic flow of CBN and metallide binder is insufficient to fill all the porosity. Addition of copper or iron is also preferred to suppress formation of brittle titanium borides.
We next consider the prior-art use of silicon as binder for CBN compacts. Silicon and its alloys with metals (other than with aluminum) have been utilized as binders for CBN principally because of their reasonably attainable melting temperatures, their hardness and abrasion resistance, their low coefficient of thermal expansion, and especially because of their lack of reactivity towards CBN.
CBN compacts utilizing silicon and silicon alloys with certain transition metals are taught by U.S. Pat. Nos. 4,401,443 to Lee et al. and 4,220,455 to St. Pierre et al., wherein a pre-compacted mass of coated CBN crystals is infiltrated with silicon or a silicon alloy. These compacts are formed at pressures far below those required for conversion of HBN to CBN. Under such conditions, binders having moderate reactivity toward CBN and hence solubility for CBN would promote the reverse transformation of CBN to HBN. No such conversion occurs with silicon; indeed the reactivity is so low that a coating (tungsten, molybdenum, or carbon, respectively) is required to enable effective bonding of the CBN grains to the silicon binder necessary. The resultant high concentrations of binder, combined with lack of direct inter-granular bonds, leads to rapid erosion in more abrasive applications.
Alloys of aluminum and silicon are taught to be effective catalysts for conversion of HBN to CBN in U.S. Pat. No. 3,959,443 to Kabayama. The specification teaches very low catalytic activity for pure silicon but substantially higher activity for alloys of silicon with aluminum, aluminum nitride, and aluminum boride.
U.S. Pat. No. 4,334,928 to Hara et al., discloses CBN compacts made with binder materials selected from carbides, nitrides, carbonitrides, borides, and silicides of group IVa, Va and VIa transition metals. Hara et al. teaches that the compact should contain less than 80 volume percent CBN. Hara also teaches that a catalyst, such as aluminum and/or silicon, may be added to the binder in a small amount. Enhanced bonding between the CBN grains and the metallide matrix is thereby achieved. X-ray diffraction shows neither free silicon nor aluminum nitride in the compact, but rather titanium-aluminum and silicontitanium compounds. The Hara et al. invention neither intends nor achieves substantial direct CBN-CBN bonding, in part due to the low concentration of CBN. Because of the low volume concentration of CBN and lack of intergranular bonding, wear resistance in abrasive applications is poor, but the excellent thermal stability of this compact makes it suitable for the hot-chip mode of cutting.
U.S. Pat. No. 4,375,517 to Watanabe et al. also discloses a high (20 to 80%) volume fraction of hard metal-based binder, in this case a cermet containing titanium carbide and/or nitride, a ferrous metal, and molybdenum or dimolybdenum carbide. As with the Hara et al. patent, addition of a very small quantity of aluminum-silicon alloy to the cermet binder is found to enhance adhesion of the dispersed CBN grains to the continuous cermet phase. Presence of ferrous metal in 3 to 20 weight percent is harmful to the thermal stability, and the high binder concentration is harmful to abrasion resistance. Thus the Watanabe et al. compact does not perform well at either of the extremes of cutting tool applications.